A gRPC rollout can leave old producers, new consumers, and intermediate services running together. Protobuf helps them exchange binary messages by identifying fields with numbers and wire types. That tolerance is narrower than application compatibility: a message can parse successfully while a consumer misunderstands its values, ignores a new instruction, or loses the distinction between absent and explicitly zero. ...
A foreign key makes a relationship enforceable during ordinary database writes. A task can reference an existing project, and the database can reject an unknown project identifier even when an import or a new application code path forgets to check it. The constraint defines referential integrity; it does not decide whether the selected project is the correct one for the business operation. ...
A zero-downtime migration is a compatibility and availability goal, not a guarantee attached to small statements. Old and new application versions may overlap while data is changing. Plan the schema, writer behavior, backfill, reader cutover, and recovery together. Dropping information is not automatically reversible, even when the preceding steps were additive. ...